Generally, a polishing process is used to polish part of the face of a casting in various shapes and patterns such as a mold, a cold forging mold and a blank mold to be as smooth as a mirror.
This polishing process plays the role of minimizing surface roughness after a milling or grinding process in order to make the surface smooth and maintain surface precision.
Equipment such as home appliances and OA (Office Automation) devices tend to have a short life cycle due to more diverse consumer demand, so the demand for molds continually increases. However, the polishing process, which is the final step of the mold manufacturing, is still conducted manually. There is designed a 6-axis multi-joint robot to automate the polishing process. However, the positioning ability of this robot is inferior to a machine tool having X, Y and Z axes, thereby inferior in precision to other finished surfaces.
As an example of the conventional polishing machine, FIG. 1 shows a machine tool having a 3-directional carrying unit. In this polishing machine, a table 3 is rotatably mounted on a main body 1, and a workpiece is fixed on the table 3. An instruction box 5 having various control buttons installed on one side of the main body 1 for controlling the machine. The signal input via the control buttons of the instruction box 5 is transmitted to a control box 7, which is provided on one side of the body and outputs a control signal to various operating units.
In addition, a pair of columns 9 are fixed to the main body 1. A pair of the columns 9 supports a X-axis carrying unit 11 which is used for lateral movement along the X-axis as in FIG. 1. A Y-axis carrying unit 13 is attached to the front side of the X-axis carrying unit 11 in the shape of a cantilever. The Y-axis carrying unit 13 has the same height as the X-axis carrying unit 11, and is used for front or back movement along the Y-direction as shown in FIG. 1.
Moreover, a Z-axis carrying unit 15 used for vertical movement along the Z-axis as in FIG. 1 is attached to one side of the Y-axis carrying unit 13. Below the Z-axis carrying unit 15, a grinding tool holder 19 is mounted so as to be rotatable at a predetermined angle. A grinding tool 17 is fixed to one side of the grinding tool holder 19.
The conventional polishing machine constructed as above is operated as follows.
If a user sends a manipulating signal through the instruction box 5, operating signals are transmitted to the X-, Y- and Z-axis carrying units 11, 13 and 15 respectively through the control box 7, thereby controlling the grinding tool 17 moving it to a desired position.
At the same time, the grinding tool holder 19 rotates at a predetermined angle so that the grinding tool 17 may more precisely contact the workpiece fixed on table 3.
If the compressed air is supplied to the grinding tool holder 19, the polishing process of the workpiece is conducted by rotation of the grinding tool 17.
In the conventional polishing machine having a configuration and operation as above, the X-axis carrying unit 11 and the Y-axis carrying unit 13 are combined in the shape of a cantilever. Thus, the Y-axis carrying unit 13 is apt to be overloaded. In particular, when the polishing process is performed on a corner of a minute curve by means of the grinding tool 17, vibration and noise may be generated.
In addition, the rotation number of the grinding tool 17 should be adjusted according to materials of the tool and the mold. However, since the grinding tool 17 is rotated by the compressed air in the conventional method, the adjustment in the number of rotations of the tool 17 is not accurate, thereby failing to perform adequate polishing work.
Moreover, in conducting the polishing process while the rotating grinding tool 17 is in contact with the surface of the workpiece, there is required to put pressure on the surface of the workpiece at a constant pressure in order to maintain constant finish of the surface. However, since the conventional machine is not equipped with a means to maintain a stable pressure, the work reliability is lower.